1. Field of the Invention
The present invention relates to an optical element in which two different voltages are applied between an electrode provided on a substrate constituting a liquid crystal cell and an electrode provided outside the substrate to control the orientation of liquid crystal molecules and in which specific optical properties can be easily adjusted.
2. Description of the Related Art
Liquid crystal has fluidity as liquid and exhibits anisotropy in electro-optical properties. The orientation of liquid crystal molecules can be controlled in various ways. The properties of the liquid crystal have been utilized, developing thin and light weight, flat-type display devices have been remarkably developed in recent years. The orientation of the liquid crystal molecules can be easily controlled if two glass plates constituting a liquid crystal element and having a transparent conductive film are surface-treated and if a voltage is externally applied. The refractive index of any liquid crystal element of this type can be continuously varied from the value it exhibits to extraordinary light to the value it exhibits to ordinary light. This is an excellent property not present in other optical materials.
Focus-variable lenses have been proposed, each of which has the effective refractive index varied (see Patent Document 1 and Non-Patent Documents 1 and 2). Glass substrates having transparent electrodes are bent by utilizing the electro-optical effect of nematic liquid crystal. The liquid crystal layer is thereby shaped like a lens, unlike in the element structure incorporated in the ordinary liquid crystal display. A voltage is applied across the electrodes, controlling the orientation of the liquid crystal molecules. The effective refractive index of the lens is thereby varied.
A method is available in which a spatial distribution of refractive index is imparted to an optical medium, thereby attaining a lens effect. Such optical media, known as SELFOC (GRIN) lenses, are commercially available. In a nematic liquid crystal cell, the liquid crystal molecules are oriented in the direction of an electric field. Methods of providing liquid crystal lenses that exhibit a spatial distribution of refractive indices have been reported (see Patent Document 2, Patent Document 3, and Non-Patent Documents 3 and 4). In these methods, an electrode having a circular pattern is used, generating an axially symmetrical non-uniform electric field, and obtaining such a liquid crystal lens by utilizing the effect of the liquid-crystal molecular orientation.
As Patent Document 4 discloses, a mesh-like macromolecule network in the liquid crystal in order to improve the properties of the liquid crystal. It is comparatively easy to modify such a lens using liquid crystal into a microlens array that comprises a number of tiny, so-called microlenses arranged two-dimensionally, thus forming a flat plate.
It has been proposed that, in a liquid crystal microlens, a pair of electrodes should be provided outside the electrode of a circular pattern to improve the lens properties (see Non-Patent Document 5). Further, a method has been proposed, in which an insulating layer is inserted between a liquid crystal layer and an electrode having a circular pattern, and the requirement that the ratio of the diameter of the circular pattern to the thickness of the liquid crystal layer should be 2:1 to about 3:1 to impart optimal properties to a microlens is mitigated (see Non-Patent Documents 6 and 7).
On the other hand, an optical apparatus has been proposed, which uses a liquid crystal element instead of a lens mirror (see Patent Document 5). In this apparatus, an imaging device detects an optical image obtained by an optical system having a focusing unit with an aberration-correcting mechanism, and the aberration is determined from a signal generated by the imaging device. A signal for correcting the aberration is generated, thereby correcting the aberration occurring in the optical system due to a sway of the atmosphere, in order to provide an optical image that is not distorted. Further, an electric-field controlled, anamorphic liquid crystal lens having an elliptical distribution of refractive index has been proposed as a lens that utilizes a liquid-crystal optical element (see Non-Patent Document 8).
Unlike the ordinary optical element, which is a passive element, these optical elements using liquid crystal can provide lenses that can adjust properties, such as focal distance, and the aberration of an optical system.
Polymerization-curable liquid crystal can be used as liquid crystal material. In this case, the liquid crystal is polymerized and is thereby cured, providing a polymer lens, after the focal distance is adjusted (see Patent Document 6).
Pat. Doc. 1: Jpn. Pat. Appln. KOKAI Publication No. 54-151854
Pat. Doc. 2: Jpn. Pat. Appln. KOKAI Publication No. 11-109303
Pat. Doc. 3: Jpn. Pat. Appln. KOKAI Publication No. 11-109304
Pat. Doc. 4: Jpn. Pat. Appln. KOKAI Publication No. 10-239676
Pat. Doc. 5: Jpn. Pat. Appln. KOKAI Publication No. 03-265819
Pat. Doc. 6: Jpn. Pat. Appln. KOKAI Publication No. 09-005695
Non-Pat. Doc. 1: S. Sato, “Liquid-crystal lens-cell with variable focal length”, Japanese Journal of Applied Physics, 1979, Vol. 18, pp. 1679-1683
Non-Pat. Doc. 2: S. Sato, “Liquid crystals and application thereof”, Sangyo Tosho Co., Ltd., Oct. 14, 1984, pp. 204-206
Non-Pat. Doc. 3: T. Nose and S. Sato, “A liquid-crystal micro lens obtained with a non uniform electric field”, Liquid Crystals, 1989, pp. 1425-1433
Non-Pat. Doc. 4: S. Sato, “The world of liquid crystal”, Sangyo Tosho Co., Ltd., Apr. 15, 1994, pp. 186-189
Non-Pat. Doc. 5: M. Honma, T. Nose and S. Sato, “Enhancement of numerical aperture of liquid crystal microlenses using a stacked electrode structure”, Japanese Journal of Applied Physics, August 2000, Vol. 39, No. 8, pp. 4799-4802
Non-Pat. Doc. 6: M. Ye and S. Sato, “Optical properties of liquid crystal lens of any size”, Preliminary reports, 49th meeting of the Applied Physics Society, March 2002, 28p-X-10, p. 1277
Non-Pat. Doc. 7: M. Ye and S. Sato, “Optical properties of liquid crystal lens of any size”, Japanese Journal of Applied Physics, May 2002, Vol. 41, No. 5, pp. L571-L573
Non-Pat. Doc. 8: Y. Yokoyama, M. Ye and S. Sato, “Electrically controllable liquid crystal anamorphic lens”, 2004 preliminary reports, meeting of the Society of Liquid Crystal, Japan, Sep. 26, 2004